Answer
There is no way to perfectly and completely understand the Trinity. But the Bible is clear that the One True God exists eternally in three Persons: Father, Son, and Spirit. We properly speak of three Persons who share the same divine essence and nature: God the Father, God the Son, and God the Holy Spirit.
Each Person of the Godhead is coequal and coeternal—no one of them is inferior to the others (John 1:1–2). Each member of the Trinity is distinct in Personhood and fulfills a different role or function.
With that foundation, we can look at who God the Spirit is.
God the Spirit is the third Person of the Trinity and proceeds from God the Father through God the Son (John 15:26). The Spirit is fully God and has eternally existed as such (Genesis 1:1–2; Hebrews 9:14). As God, the Spirit possesses all the divine attributes, including omniscience (1 Corinthians 2:10–11), omnipotence (Luke 1:35), and omnipresence (Psalm 139:7–10). God the Spirit is of the same essence as God the Father and God the Son but distinct in Personhood from them and submits Himself to them (John 14:26, 16:7).
God the Spirit is a person and not simply a force. In other words, He has personality—a mind, emotions, and a will. The Spirit has distinctive traits and character, including the ability to encourage (Acts 9:31), comfort (John 14:26), and be lied to (Acts 5:3).
God the Spirit has worked throughout redemptive history in a variety of ways to enact the will of God. He is the power of God and was instrumental in creating the heavens, the earth, and humanity (Genesis 1—2). God the Spirit guided the Israelites as they journeyed to the Promised Land (Isaiah 63:11–14). Today, He guides believers as they await the return of God the Son (John 16:13). God the Spirit temporarily and situationally indwelled certain people in the Old Testament (predominantly Israelite kings and prophets) to empower them for service to God (1 Samuel 16:13; Micah 3:8). Today, God the Spirit permanently indwells believers to empower them for service to God (1 Corinthians 3:16; Ephesians 2:22). He convicts the world of sin, righteousness, and judgment (John 16:8). Throughout history, God the Spirit “carried along“ the authors of the biblical texts to ensure the Word of God was established as trustworthy and true (2 Peter 1:20–21; cf. 1 Corinthians 2:12–13; 2 Timothy 3:16–17).
God the Spirit also intercedes for believers before God (Romans 8:27). He reminds them of everything that God the Son taught and baptizes them into His body (John 14:16–26; 1 Corinthians 12:13–14). Through this advocacy and baptism, God the Spirit works to unify and sanctify believers to conform them to the image of Christ (Romans 8:5–17, 29). This lifelong process of sanctification involves guiding and directing believers in their affairs (Acts 10:19–20), gifting them with certain talents and abilities for service in the church (Romans 12:6–8; 1 Corinthians 12:4–11), searching their hearts and helping them in their weaknesses (Romans 8:26–27), unifying them in their differences (1 Corinthians 12:13), and imparting divine wisdom (Proverbs 2). By and through the power of God the Spirit, believers are washed, sanctified, and justified in the name of God the Son (1 Corinthians 6:11).
